Catalyst based on palladium, gold, alkali metal and lanthanoid and process for preparing vinyl acetate

ABSTRACT

A process for the vapor phase preparation of vinyl acetate in the presence of a catalyst of 0.2 to 1.3% by weight of gold or a gold compound, 0.3 to 10% by weight of an alkali metal compound, 0.5 to 2.0% by weight of a palladium compound and 0.01 to 1% by weight of a lanthanoid compound.

[0001] The present invention relates to a catalyst which comprisespalladium and/or its compounds, gold and/or its compounds, alkali metalcompounds and at least one lanthanoid metal and/or its compounds, and toits use for preparing vinyl acetate from acetic acid, ethylene andoxygen or oxygen-containing gases.

[0002] It is known that ethylene can be converted in the gas phase withacetic acid and oxygen or oxygen-containing gases onpalladium/gold/alkali metal-containing fixed bed catalysts into vinylacetate.

[0003] The palladium/gold/alkali metal-containing catalysts have aparticular noble metal distribution, with the noble metals being presentin a shell on the carrier particles, while the core of the particles issubstantially free of noble metals. Catalysts with this noble metaldistribution are distinguished by an increased specific productivity (gof vinyl acetate/g of noble metal). The noble metal compound in shellform is achieved by impregnation and subsequent precipitation of thenoble metals using alkaline compounds.

[0004] The process disclosed in U.S. Pat. No. 4,048,096 for preparingpalladium, potassium and gold-containing catalysts entails initialimpregnation of the carrier material with an aqueous solution whichcomprises a mixture of palladium and gold salts. The metal salts arethen converted by treatment with alkalis into water-insoluble compoundsand are fixed on the carrier material in this way. Subsequent treatmentwith a reducing agent reduces the palladium and gold compounds to thecorresponding metals. Finally, the carrier material loaded withpalladium and gold is treated with an alkali metal acetate solution anddried. The impreg- nation step with the aqueous solution containingpalladium and gold salts is characterized by the volume of theimpregnation solution corresponding to the pore volume of the carriermaterial. The resulting catalyst has a shell structure in whichpalladium and gold are dispersed in a shell thickness of about 0.5millimeter over the surface of the carrier material.

[0005] U.S. Pat. No. 3,775,342 also discloses a process for preparingpalladium, potassium and gold-containing catalysts by impregnation witha solution of palladium and gold salts, by subsequent treatment with analkali solution, which results in water-insoluble palladium and goldcompounds precipitating on the carrier, and by subsequent reduction ofthe metal compounds to the corresponding noble metals. Treatment of thecarrier material with an alkali metal acetate solution can take placebefore or after the reduction step.

[0006] U.S. Pat. No. 5,185,308 discloses a palladium, potassium andgold-containing shell catalyst in which the noble metals are dispersedin a shell thickness of 1 millimeter over the carrier material. Theknown catalyst has a ratio of gold to palladium in the range from 0.6 to1.25 by weight.

[0007] It is further known to prepare a palladium, potassium andgold-containing shell catalyst by washing a carrier material, which hasbeen provided with a binder, for example an alkali metal or alkalineearth metal carboxylate, before the impregnation with an acid, andtreating with a base after the impregnation (EP-A-0 519 435).

[0008] In the process disclosed in U.S. Pat. No. 5,332,710 for preparinga palladium, gold and potassium-containing shell catalyst, the carrierimpregnated with an aqueous palladium and gold salt solution is immersedin an aqueous fixing solution containing sodium hydroxide or potassiumhydroxide and agitated therein for at least 0.5 h.

[0009] It has now been found, surprisingly, that catalysts of this typecan be distinctly improved by adding at least one lanthanoid metaland/or a lanthanoid metal compound, i.e. provide a higher space-timeyield with identical or higher selectivity for vinyl acetate.

[0010] The invention accordingly relates firstly to a process forpreparing vinyl acetate in the gas phase from ethylene, acetic acid andoxygen or oxygen-containing gases on a catalyst which comprisespalladium and/or its compounds, gold and/or its compounds, and alkalimetal compounds on a carrier, wherein the catalyst additionallycomprises at least one lanthanoid metal and/or its compounds.

[0011] The invention secondly relates to a catalyst which comprisespalladium and/or its compounds, gold and/or its compounds, and alkalimetal compounds on a carrier, wherein the catalyst additionallycomprises at least one lanthanoid metal and/or its compounds.

[0012] The procedure for preparing the catalysts according to theinvention is preferably as follows (U.S. Pat. No. 3,775,342, U.S. Pat.No. 4,048,096, U.S. Pat. No. 5,332,710):

[0013] (1) First the carrier particles are impregnated one or more timesby being intimately mixed with at least one solution of at least onesalt of the elements palladium and gold, and of at least one salt of atleast one lanthanoid metal.

[0014] (2) The pretreated carrier is treated with a fixing solution withan alkaline reaction, which results in the noble metals and thelanthanoid metals being precipitated in the form of water-insolublecompounds on the surface of the carrier particles, and thus being fixed.

[0015] (3) The noble metal compounds deposited on the carrier particlesare reduced to the corresponding metals by treatment with a reducingagent. A noble metal shell doped with at least one lanthanoid metal isproduced in this way on the surface of the carrier particles.

[0016] (4) Interfering anions are removed by washing the treatedcatalyst.

[0017] (5) The treated catalyst is dried at not above 150° C.

[0018] (6) The dried carrier is treated with a solution which containsat least one alkali metal compound.

[0019] (7) Finally, the treated carrier is dried at not above 150° C.

[0020] The procedure in step (1) can also be to apply the salt solutionscontaining catalytically active substances to the carrier by single ormultiple spraying on, vapor deposition or immersion.

[0021] The term “lanthanoid metals” means the 14 rare earth elementscerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium andlutetium, and the elements scandium, yttrium and lanthanum because theirchemical behavior resembles that of the rare earth elements.

[0022] Suitable carriers are the known inert carrier materials such assilica, alumina, aluminosilicates, silicates, titanium oxide, zirconiumoxide, titanates, silicon carbide and carbon. Particularly suitablecarriers of this type are those with a specific surface area of 40 to350 m²/g (measured by the BET method) and an average pore radius of 50to 2000 Å (Angstrom) (measured by mercury porosimetry), especiallysilica (SiO₂) and SiO₂/Al₂O₃ mixtures. These carriers can be used in anyform such as, for example, in the form of beads, tablets, rings, starsor particles of other shapes, with a diameter or length and thicknessgenerally of 3 to 9 mm.

[0023] Carriers of these types can be prepared, for example, fromaerogenic SiO₂ or an aerogenic SiO₂/Al₂O₃ mixture which can be prepared,for example, by flash hydrolysis of silicon tetrachloride or a silicontetrachloride/aluminum trichloride mixture in an oxyhydrogen flame (U.S.Pat. No. 3,939,199).

[0024] Suitable solvents for the palladium, gold, alkali metal andlanthanoid metal salts are all compounds in which the selected salts aresoluble and which can easily be removed again after the impregnation bydrying. Suitable for the acetates are, in particular, unsubstitutedcarboxylic acids having 2 to 10 carbon atoms such as acetic acid,propionic acid, n- and iso-butyric acid and the various valeric acids.Among the carboxylic acids, acetic acid is preferred because of itsphysical properties and also for economic reasons. Water is particularlysuitable for the chlorides and chloro and acetato complexes. Additionaluse of another solvent is expedient if the salts are insufficientlysoluble in acetic acid or in water. Thus, for example, palladiumchloride can be dissolved considerably better in an aqueous acetic acidthan in glacial acetic acid. Suitable additional solvents are thosewhich are inert and are miscible with acetic acid or water. Those whichmay be mentioned as additions for acetic acid are ketones such asacetone and acetylacetone, also ethers such as tetrahydrofuran ordioxane, but also hydrocarbons such as benzene.

[0025] It is possible to apply a plurality of salts of palladium, gold,alkali metal and the particular lanthanoid metal, but generally exactlyone salt of each of these elements is applied.

[0026] The elements palladium and gold which are to be applied in eachcase in the procedure of step (1), and the lanthanoid metal to beapplied in each case, can be applied in the form of salt solutions,singly or else in any suitable combination in any suitable sequence,preferably using a single solution which contains these elements to beapplied in the form of salts. It is particularly preferred to use asingle solution which contains exactly one salt of each of theseelements to be applied.

[0027] This solution preferably contains a salt of a single lanthanoidmetal, but it is also possible to use a solution which contains one saltof each of different lanthanoid metals.

[0028] Where the following speaks generally of “the solution of thesalts”, the same applies analogously to the case where a plurality ofsolutions are employed in sequence, each of which contains only part ofthe totality of salts to be applied, in which case the total of theindividual parts amounts to the total quantity of the salts to beapplied to the carrier.

[0029] For the procedure of step (1), the solution of the salts isapplied to the carrier particles by impregnating the latter one or moretimes with this solution, employing the total volume of the solution allat once or divided into two or more part-volumes. However, it isexpedient to use the total volume of the salt solution all at once, sothat the carrier particles are impregnated with the required amount ofelements to be applied by a single impregnation, in which case dryingcan follow immediately. In the case of impregnation sequentially with aplurality of part-volumes, drying is carried out immediately after eachimpregnation.

[0030] “Immediate” drying means in this connection that drying theimpregnated particles must start without delay. It is generallysufficient for this case to start drying the particles no later thanhalf an hour after the end of an impregnation.

[0031] The impregnation of the carrier particles with the solution ofthe salts to be applied is carried out by covering the carrier particleswith the solution and, where appropriate, then pouring off or filteringoff excess solution. It is advantageous, with regard to losses ofsolution, to employ only the quantity of solution corresponding to theintegral pore volume of the catalyst carrier.

[0032] It is expedient to mix the carrier particles intimately duringthe impregnation, for example in a rotating or agitated flask or amixing drum, in which case drying can follow immediately. The speed ofrotation or intensity of the agitation must, on the one hand, besufficient to ensure good mixing and wetting of the carrier particlesbut must, on the other hand, not be so great that there is considerableabrasion of the carrier material.

[0033] The solution of the salts should have a temperature which is highenough to prevent the salts precipitating during the application to thecarrier. The temperature should, however, generally not be much above70° C. in order to avoid excessive evaporation of the solvent anddecomposition of the noble metal compounds.

[0034] The treatment of the carrier particles impregnated in step (1)with a solution with an alkaline reaction converts the salts of theapplied elements into water-insoluble compounds, and they are thus fixedto the surface of the carrier (step (2)).

[0035] Examples of fixing solutions which can be used are aqueoussolutions with an alkaline reaction. Examples of such solutions areaqueous solutions of alkali metal silicates, alkali metal carbonates andbicarbonates or alkali metal hydroxides.

[0036] An aqueous solution of the alkali metal hydroxides, in particularpotassium or sodium hydroxide, is preferred. Aqueous solutions whichcontain boron compounds can also be used as solutions with an alkalinereaction. Particularly suitable in this case are aqueous solutions ofborax, potassium tetraborate or mixtures of alkali metal hydroxidesolution and boric acid. The alkaline solution may have bufferingproperties.

[0037] The amount of the compound with an alkaline reaction present inthe fixing solution is expediently such that it is at least sufficientfor stoichiometric conversion of the applied palladium, gold andlanthanoid metal salts into water-insoluble compounds.

[0038] However, it is also possible to use an excess of the compoundwith an alkaline reaction present in the fixing solution, the excessgenerally being 1 to 10 times the amount required by the stoichiometry.

[0039] The volume of the fixing solution must be at least sufficient tocover the impregnated carrier completely with the fixing solution. Thefixing preferably takes place by the rotation immersion techniquedisclosed in U.S. Pat. No. 5,332,710, which is incorporated herein byreference. This technique comprises agitating the carrier which iscompletely covered by the fixing solution by rotation from the start ofthe treatment with the fixing solution.

[0040] Every type of rotation or similar treatment which keeps thecarrier particles agitated can be used, because the exact manner is notcritical. The intensity of the agitation is important, however. Itshould be sufficient for the entire surface area of the impregnatedcarrier to be wetted uniformly with the alkaline fixing solution.

[0041] The treated carrier is then left to stand in the fixing solutionat room temperature for up to 16 hours in order to ensure that theapplied palladium, gold and lanthanoid metal salts are completelyprecipitated in the form of water-insoluble compounds on the catalystcarrier.

[0042] The reaction on the carrier can, however, also be carried out atelevated temperature, for example at 70° C.

[0043] After the fixation is complete, the supernatant fixing solutionis poured away. This can be followed, where appropriate, by washing thetreated carrier in order to remove the soluble compounds present on thetreated carrier, for example the alkali metal chlorides liberated in thefixing step and any excess which is present of the compound with analkaline reaction present in the fixing solution, by washing.

[0044] For this purpose, the treated carrier is continuously washed withthe washing liquid, preferably with running demineralized water, at roomtemperature. The washing is continued until interfering anions, forexample chlorides, are substantially removed from the carrier. The moistimpregnated catalyst carrier can then be dried, which is expedient ifthe subsequent reduction of the deposited noble metal compounds to thecorresponding metals (step (3)) is carried out in the gas phase.

[0045] Reduction of the water-insoluble compounds fixed on the catalystcarrier to the corresponding metals can be carried out with a gaseousreducing agent (step (3)).

[0046] The reduction temperature is generally between 40 and 260° C.,preferably between 70 and 200° C. It is generally expedient to use forthe reduction a reducing agent which is diluted with inert gas andcontains 0.01 to 50% by volume, preferably 0.5 to 20% by volume, ofreducing agent. It is possible to use as inert gas, for example,nitrogen, carbon dioxide or a noble gas. Examples of suitable reducingagents are hydrogen, methanol, formaldehyde, ethylene, propylene,isobutylene, butylene or other olefins. The reduction can also becarried out in liquid phase at a temperature from 0° C. to 90° C.,preferably from 15 to 25° C. Examples of reducing agents which can beused are aqueous solutions of hydrazine, formic acid or alkali metalborohydrides, preferably sodium borohydride. The amount of reducingagent depends on the amount of the noble metals; the reductionequivalent should be at least equal to oxidation equivalent in quantity,but larger amounts of reducing agent are not harmful.

[0047] It is essential to select the reduction conditions in thereduction step so that the fixed water-insoluble noble metal compoundsare reduced to the corresponding noble metals. It is, on the other hand,immaterial whether the fixed water-insoluble lanthanoid metal compoundsare also converted under the selected reduction conditions into thecorresponding lanthanoid metals, because it is not critical for thesuitability of the novel catalysts for preparing vinyl acetate whetherthe lanthanoid metals are present as elements and/or their compounds inthe noble metal shell of the novel catalysts.

[0048] If no washing step takes place after the fixation is complete(step (2)), or if the reduction takes place with an aqueous solution ofa reducing agent, the treated catalyst carrier must, after the reductionis complete, be washed several times to remove interfering compounds,for example to remove chloride residues derived from the impregnationstep and released due to the fixation and reduction of the noble metals(step (4)).

[0049] For this purpose, the treated carrier is washed continuously withthe washing liquid, preferably with running demineralized water, at roomtemperature until interring anions, for example chlorides, are removed.

[0050] If an aqueous solution of a reducing agent is used in step (3),residues of the reducing agent used can also be removed with the washingstep.

[0051] The catalyst is then dried at temperatures not exceeding 150° C.(step (5)).

[0052] In step (6), the dried catalyst carrier is then treated,preferably impregnated, one or more times with a solution of an alkalimetal compound, the total volume of the solution being employed all atonce or divided into part-volumes. However, it is expedient to use thetotal volume of the solution all at once, so that the carrier particlesare impregnated with the required amounts of alkali metal compound to beapplied by a single impregnation. The volume of the solution of thealkali metal compound is, in the case of single or multipleimpregnation, generally between 60 and 110%, preferably between 80 and100%, of the pore volume.

[0053] The solution of the alkali metal compound can also be applied tothe carrier by single or multiple spraying on, vapor deposition orimmersion.

[0054] After the treatment with a solution of an alkali metal compound,the catalyst carrier is finally dried at no higher than 150° C. (step(7)).

[0055] The alkali metal compound is used in an amount such that thecatalyst carrier contains 0.1 to 10% by weight of alkali metal after thedrying. The drying of the treated catalyst carrier to be carried out insteps (5) and (7) takes place in a stream of hot air or in a stream ofinert gas, for example in a stream of nitrogen or carbon dioxide. Thetemperature during this drying should generally be 60 to 150° C.,preferably 100 to 150° C. Drying is moreover carried out, whereappropriate, under reduced pressure, generally from 0.01 Mpa to 0.08Mpa.

[0056] If the drying forms part of step (1) and, where appropriate, theother steps, the procedure is the same.

[0057] The finished shell catalysts containing palladium, gold, alkalimetal and at least one lanthanoid metal have the following metalcontents: Palladium content: generally 0.5-2.0% by weight, preferably0.6-1.5% by weight; Gold content: generally 0.2-1.3% by weight,preferably 0.3-1.1% by weight; Alkali metal content: generally 0.3-10%by weight, and potassium is preferably used. Potassium content:generally 0.5-4.0% by weight, preferably 1.5-3.0% by weight; Lanthanoidmetal content: generally 0.01-1% by weight, preferably 0.05-0.5% byweight.

[0058] If more than one lanthanoid metal is used to dope the palladium,gold and alkali metal-containing shell catalysts, the term “lanthanoidmetal content” means the total content of all the lanthanoid metalspresent in the finished catalyst. The stated percentages always relateto the amounts of the elements palladium, gold, alkali metal andlanthanoid metal present in the catalyst, based on the total mass of thecatalyst (active elements plus anions plus carrier material).

[0059] Suitable salts are all salts of palladium, gold, an alkali metaland a lanthanoid element which are soluble; the acetates, the chlorides,and the acetato and chloro complexes are preferred. However, in the caseof interfering anions such as, for example, in the case of chlorides, itmust be ensured that these anions are substantially removed before useof the catalyst. This takes place by washing the doped carrier, forexample with water, after, for example, the palladium and gold whichhave been applied as chloride have been converted into an insolubleform, for example through the fixation with compounds having an alkalinereaction and/or by reduction (steps (2) and (3)).

[0060] Particularly suitable salts of palladium and gold are chloride,chloro complexes and carboxylates, preferably the salts of aliphaticmonocarboxylic acids having 2 to 5 carbon atoms, for example theacetate, propionate or butyrate. Further suitable examples are thenitrate, nitrite, oxide hydrate, oxalate, acetylacetonate oracetoacetate. Because of the good solubility and availability, preferredpalladium and gold salts are in particular the chlorides and chlorocomplexes of palladium and gold.

[0061] The alkali metal compound preferably employed is at least onesodium, potassium, rubidium or cesium compound, in particular apotassium compound. Particularly suitable compounds are carboxylates, inparticular acetates and propionates. Compounds which are converted underthe reaction conditions into the alkali metal acetate, such as, forexample, the hydroxide, the oxide or the carbonate, are also suitable.

[0062] The lanthanoid metal compound employed is preferably at least onepraseodymium, neodymium, samarium, europium or dysprosium compound.However, it is also possible to employ mixtures of these compounds.

[0063] The chlorides, nitrates, acetates and acetylacetonates areparticularly suitable as lanthanoid metal compound.

[0064] In the novel catalysts, the noble metals and the particularlanthanoid metals and/or their compounds are applied in a shell on thecarrier particle.

[0065] Vinyl acetate is generally prepared by passing acetic acid,ethylene and oxygen-containing gases at temperatures from 100 to 220°C., preferably 120 to 200° C., under pressures from 0.1 to 2.5 Mpa,preferably 0.1 to 2.0 Mpa, over the finished catalyst, it being possibleto circulate unreacted components. It is also advantageous in somecircumstances to dilute with inert gases such as nitrogen or carbondioxide. Carbon dioxide is particularly suitable for the dilutionbecause it is formed in small amounts during the reaction.

[0066] With the same reaction conditions it is possible with the aid ofthe novel catalysts to prepare more vinyl acetate per reactor volume andtime with, at the same time, improved selectivity by comparison withknown catalysts.

[0067] This facilitates the workup of the resulting crude vinyl acetatebecause the vinyl acetate content in the gas discharged from the reactoris higher, which further results in a saving of energy in the workuppart. A suitable workup is described, for example, in U.S. Pat. No.5,066,365.

[0068] If, on the other hand, it is wished to keep the space-time yieldconstant, it is possible to reduce the reaction temperature and thuscarry out the reaction more selectively, with the same totalproductivity, in which case there is a saving of precursors. This isalso associated with a decrease in the amount of carbon dioxide, whichis formed as byproduct and therefore must be removed, and in the loss ofentrained ethylene which is associated with this removal. In addition,this procedure results in an increase in the useful life of thecatalyst.

[0069] The following examples are intended to illustrate the inventionbut do not restrict it. The percentages of the elements palladium, gold,potassium and of the lanthanoid element are percent by weight based onthe total mass of the catalyst.

[0070] The catalyst carrier used was the SiO₂ carrier available fromSüd-Chemie with the name KA 160 in the form of beads with a diameter of5 mm. The pore volume of 1 l of carrier was 335 ml.

EXAMPLE 1

[0071] 5.37 g (=0.0164 mol) of potassium tetrachloropalladate, 3.36 g(0.0089 mol) of potassium tetrachloroaurate and 0.74 g (0.0018 mol) ofpraseodymium trinitrate pentahydrate were weighed out together anddissolved in 90 ml of demineralized water (solution volume =100% of thepore volume). With gentle agitation, this solution was completelyadsorbed onto 147.5 g of the carrier material at room temperature. Toprecipitate insoluble palladium, gold and praseodymium compounds, whichleads to formation of a noble metal shell, the pretreated carrier wasmixed with a solution of 3.1 g of sodium hydroxide in 300 ml ofdemineralized water. Immediately after addition of the alkaline fixingsolution, the carrier was agitated in a rotary evaporator rotating at arate of 5 revolutions per minute (rpm) for a period of 2.5 hours. Tocomplete the precipitation, the mixture was left to stand at roomtemperature for a period of 14 hours. The supernatant solution was thenpoured off, and the mixture was washed with demineralized water untilfree of chloride. A water flow rate of 200 ml/minute for approximately 5hours was necessary for this. To check for freedom from chloride, asilver nitrate solution was added to the washing water and it wasexamined for silver chloride precipitation. The catalyst wassubsequently dried at a temperature of 100° C. for a period of 2 hours.It was then reduced with a gas mixture consisting of 5% by volumeethylene and 95% by volume nitrogen, passing this gas mixture over thecatalyst at a temperature of 150° C. for a period of 5 hours. Thereduced catalyst was then impregnated with a solution of 10 g ofpotassium acetate in 75 ml of demineralized water (solution volume=83%of the pore volume) in portions and dried with hot air at a temperatureof 100° C. for a period of 2 hours.

[0072] The finished catalyst contained 1.1% by weight Pd, 1.1% by weightAu, 2.5% by weight K and 0.16% by weight Pr.

EXAMPLE 2

[0073] The procedure was analogous to that of Example 1 but thelanthanoid metal compound used was 0.71 g (0.0017 mol) of samariumtrinitrate pentahydrate in place of praseodymium trinitratepentahydrate.

[0074] The finished catalyst contained 1.1% by weight Pd, 1.1% by weightAu, 2.5% by weight K and 0.16% by weight Sm.

EXAMPLE 3

[0075] The procedure was analogous to that of Example 1 but 0.7 g(0.0016 mol) of europium trinitrate pentahydrate was used as lanthanoidmetal compound.

[0076] The finished catalyst contained 1.1% by weight Pd, 1.1% by weightAu, 2.5% by weight K and 0.15% by weight Eu.

EXAMPLE 4

[0077] The procedure was analogous to that of Example 1 but 0.34 g(0.0008 mol) of neodymium trinitrate pentahydrate was used as lanthanoidmetal compound.

[0078] The finished catalyst contained 1.1% by weight Pd, 1.1% by weightAu, 2.5% by weight K and 0.07% by weight Nd.

EXAMPLE 5

[0079] The procedure was analogous to that of Example 1 but 0.3 g(0.0008 mol) of dysprosium trichloride hexahydrate was used aslanthanoid metal compound.

[0080] The finished catalyst contained 1.1% by weight Pd, 1.1% by weightAu, 2.5% by weight K and 0.08% by weight Dy.

EXAMPLE 6

[0081] The procedure was analogous to that of Example 5 but 0.6 g(0.0016 mol) of dysprosium trichloride hexahydrate was used.

[0082] The finished catalyst contained 1.1% by weight Pd, 1.1% by weightAu, 2.5% by weight K and 0.16% by weight Dy.

COMPARATIVE EXAMPLE 1a

[0083] The procedure was as in Example 1 but no lanthanoid metal saltswere added to the impregnation solution containing potassiumtetrachloropalladate and potassium tetrachloroaurate.

[0084] The finished catalyst contained 1.1% by weight Pd, 1.1% by weightAu and 2.5% by weight K.

[0085] The novel catalysts prepared as in Examples 1-6, and the knowncatalyst prepared as in Comparative Example 1a, were tested in a Bertyreactor. The average temperature of the jacket of the Berty reactor waschosen so that a constant oxygen conversion of 45% was observed.

[0086] The results are to be found in the table. Space-time CO₂ Exampleyield selectivity 1 793 8.97 2 780 9.23 3 802 8.79 4 726 8.50 5 733 9.06 722 9.3 Comparative Example 1a 683 10.9

[0087] It was found, surprisingly, that even small additions oflanthanoid metals to the known palladium, gold and potassium-containingcatalysts distinctly improve the CO₂ selectivity and the productivity(space-time yield) of these catalysts in preparing vinyl acetate.

1. A process for preparing vinyl acetate in the gas phase from ethylene,acetic acid and oxygen or oxygen-containing gases on a catalyst whichcomprises palladium and/or its compounds, gold and/or its compounds, andalkali metal compounds on a carrier, wherein the catalyst additionallycomprises at least one lanthanoid metal and/or its compounds.
 2. Theprocess as claimed in claim 1, wherein the catalyst comprises at leastone potassium compound.
 3. The process as claimed in claim 1 or 2,wherein the catalyst comprises 0.01% by weight to 1% by weight oflanthanoid metal, based on the total mass of the catalyst.
 4. Theprocess as claimed in any of claims 1 to 3, wherein the catalystcomprises 0.05% by weight to 0.5% by weight of lanthanoid metal, basedon the total mass of the catalyst.
 5. The process as claimed in any ofclaims 1 to 4, wherein the lanthanoid metal is praseodymium, samarium,europium, neodymium or dysprosium.
 6. A catalyst which comprisespalladium and/or its compounds, gold and/or its compounds and alkalimetal compounds on a carrier, wherein the catalyst additionallycomprises at least one lanthanoid metal and/or its compounds.
 7. Acatalyst as claimed in claim 6, wherein the catalyst comprises at leastone potassium compound.
 8. A catalyst as claimed in claim 6 or 7,wherein the catalyst comprises 0.01% by weight to 1% by weight oflanthanoid metal, based on the total mass of the catalyst.
 9. A catalystas claimed in any of claims 6 to 8, wherein the catalyst comprises 0.05%by weight to 0.5% by weight of lanthanoid metal, based on the total massof the catalyst.
 10. A catalyst as claimed in any of claims 6 to 9,wherein the lanthanoid metal is praseodymium, samarium, europium,neodymium or dysprosium.